1. Field of the Invention
The present invention relates to an encoder for the conversion of a digital input signal defined as a code of thermometric or cyclic type, each bit of which is applied to an input of the encoder, so as to obtain a binary-encoded output signal, which encoder includes a set of n successive Exclusive OR gates, with two inputs and one output, each Exclusive OR gate having an input connected to an input of the gate of neighbouring rank as well as to an input of the encoder, and includes an encoding matrix with n rows at input and a plurality of pairs of columns at output, each of the n successive rows being connected to an output of one of the successive Exclusive OR gates, each of the columns being linked to a reference voltage terminal via a current source and the columns of one pair delivering a differential output for one bit of the binary output signal, in which encoding matrix active coupling between a specified row and a specified column is effected, when required, by a transistor the base of which is connected to the said specified row, the emitter of which is connected to the said specified column and the collector of which is connected to a supply voltage terminal.
2. Description of the Related Art
Such an encoder is known from the article entitled "8 bit 100 MHz full-Nyquist A/D Converter" published in IEEE Journal of Solid-State Circuits, Vol. 23, n.degree. 6 Dec. 1988, pp. 1334-1344. A description of this encoder produced in bipolar technology is particularly described in connection with FIGS. 7 and 13 of this publication.
A major application of a binary encoder of the type indicated above is in analogue/digital converters. In this application, a well-known technical problem is posed by the presence of logic decision errors at the output of the set of Exclusive OR gates.
Measurement of the value of an analogue signal by comparison with the voltages logged on a scale of resistances ought normally to provide a thermometric digital signal, the successive bits of which contain just a single transition between a group of successive ranked-bits at 1 and a remaining group of successive ranked bits at 0. At the output of the set of Exclusive OR gates, a single output at 1 ought therefore to be noted normally.
As a result of slight imperfections in the production of the converters, decision errors are observed, particularly when the frequency of the analogue signal to be measured is increased. A decision error, hereafter called a simple error, consists in the bits of the thermometric signal observed at a given instant containing a (1 . . . 1), 0, 1, 0, (0 . . . 0) transition.
The set of Exclusive OR gates therefore yields in this case a signal at 1 on three outputs of successive ranks, instead of a single output. Other errors which are more serious since they involve more than three successive outputs from the set of gates, are also possible, but they are markedly less frequent than the so-called simple errors and ought to be avoided through more careful construction of the converter.